Prior disposable biomedical electrodes ("snap connector electrodes") were generally in the form of a pressure sensitive adhesive pad, with a snap connector extending through the surface of the pad so that a snap stud presented itself on the upper surface, while a metal plated conductive member presented itself on the under surface. The conductive metallic layer of this sensing element comprised silver, tin, or the salts of silver or tin. A sponge, saturated with electrolyte, was placed over the conductive member on the under surface, while leaving a border of pressure sensitive adhesive to make contact with the skin. The entire assembly was then covered with a protective carrier, or liner, which would be discarded prior to use. U.S. Pat. Nos. 3,805,769, 3,834,373, 3,923,042, 3,976,055, and 3,993,049 illustrate electrodes of this type. While possessing advantages over reusable needle or suction type electrodes, these early disposable electrodes were not only relatively expensive, but also tended to dry out during storage, thus limiting their useful shelf life.
More economical types of electrodes utilizing less expensive integral-connecting tabs ("tab type electrodes") have been disclosed in several patents, including U.S. Pat. Nos. 4,125,110, 4,365,634, 4,524,087, 4,543,958, and 4,674,512. However, construction of these products, especially those which use tin as a conductive layer, is such that they do not always comply with all of the standards established by governmental regulatory agencies and medical industry groups for such electrodes. In particular, the Association for the Advancement of Medical Instrumentation (AAMI) has set forth standards for various electrical characteristics of disposable biomedical electrodes. The specification for defibrillation recovery characteristics, which describes certain time-related, electrical dissipation properties of the electrode following repeated electrical shocks of defibrillation currents, is especially difficult for many of the above "tab-type" electrodes to meet. Thus, use of the tab type electrode would invite the possibility of an inordinate, life-threatening delay in obtaining electrocardiographic data following defibrillation. This, of course, severely limits the usefulness of such electrodes in a critical care environment. Accordingly, many of these products bear a caution label that they are not to be used where defibrillation is a possibility.
U.S. Pat. No. 4,852,571 addresses some of the shortcomings of these non-snap, tab-type electrodes, and discloses a disposable electrode which passes the electrical requirements as specified by AAMI. However, the '571 design requires two separate layers of conductive inks, comprising a "discontinuous layer" of silver/silver chloride ink over a layer of carbon ink, which must be applied in two separate steps. This dual step ink application requirement increases cost and decreases process control.
Electrodes using solventless gels which are both conductive and adhesive are disclosed in several patents including U.S. Pat. Nos. 4,524,087, 4,391,278, and 4,125,110. Such gels, also known in the art as "hydrogels", utilize either a thermal or an ultraviolet photo curing process in order to convert the conductive-adhesive hydrogel into its final form. However, in most cases, the cured hydrogel must first be laminated to the other electrode materials and then die cut into the desired electrode form. The lamination and die cut requirements present significant economical disadvantages. To avoid these disadvantages, "hot melt" type of adhesives have been introduced. While hot melt adhesives may be applied directly to the conductive ink in a hot melt process (thus avoiding lamination), their utility has thus far been limited since most hot melt adhesives require application temperatures in excess of 250.degree. F., thereby excluding water based hot melts from consideration.
In addition to problems arising from the designs of the single prior art electrode, problems have also arisen in the deployment of electrode sets. In procedures such as ECG/EKG monitoring or stress testing, electrodes are placed at different sites on the human body. However, since the human body exhibits different skin impedance values at various sites, the electrode-to-skin impedance values at such sites can vary widely. If uncompensated, these variances may provide an inconsistent baseline to the monitoring equipment and generate inaccurate readings.
Lastly, experience has shown that different brand or models of monitors do not necessarily produce consistent results from the same electrode. Similar to the skin impedance problem discussed above, this phenomenon may also produce inaccurate readings.
In sum, the prior art possesses the following shortcomings:
a) snap connectors which were expensive; PA1 b) tab type electrodes which had long defibrillation recovery times; PA1 c) electrolyte adhesives which dried out; PA1 d) hydrogel adhesives which required lamination prior to die cutting; PA1 e) hot melt adhesives which could not be water base; PA1 f) electrodes which did not compensate for impedance variances; and PA1 g) bilayered inks which required dual step ink application. PA1 1) the impedances of distinctly identifiable electrodes may vary within a set thus compensating for the diverse impedances at different body sites or the specific requirements for certain monitoring equipment, PA1 2) the conductive ink layer may be an inexpensive homogeneous blend of inks which can be coated directly to a non-conductive backing material without the need for a lamination step, PA1 3) the conductive adhesive layer can be coated without lamination to the non-conductive backing layer/conductive ink layer and at a temperature suitable for water based adhesives, and PA1 4) the set can be produced in a single pass through an assembly line.
Furthermore, because all of the above described electrodes required multiple manufacturing steps, the problems associated with process control are significant.
Thus, there exists a need for a safe, economical electrode or set of electrodes which can compensate for impedance variances and can be economically manufactured, preferably in a continuous process.